Chapter 5.1 – Igneous Rocks
• Magma – molten rock below Earth’s surface
• Lava – magma that flows out onto the surface
• Igneous rocks – rocks that form when magma
or lava cools and crystallizes
– Rocks must be heated up between 800-1200 C
before they melt
Composition of Magma
• Materials in magma
– Molten rock
– Dissolved gases
– Mineral crystals
• Major elements in magma
– Oxygen, silicon, aluminum, iron, magnesium,
calcium, potassium, sodium
– Silicon is the most abundant in magma
Types of Magma
• Basaltic
– 45-52% silica
• Andesitic
– 52-66% silica
• Rhyolitic
– More than 66% silica
Magma Formation
• Formed in two ways
– Melting of Earth’s crust
– Melting within the mantle
• Four factors involved in formation
– Temperature
– Pressure
– Water content
– Mineral content
Magma Formation
• Temperature increases as you go deeper into
the Earth crust
– Change in temperature is called geothermal
gradient
• Pressure increases with depth because of
weight of rocks
• Water content changes melting point of rocks
Mineral Content
• Different minerals have different melting
points
– Basalt (made from olivine, calcium feldspar, and
pyroxene) melts at higher temperatures than
granite (made from quartz and potassium
feldspar)
– Rocks rich in iron and magnesium melt at higher
temperatures
Mineral Content
• Not all parts of a rock melt at the same
temperature
– This is why magma is a mix of molten rock and
crystals
• This process is called partial melting
• N.L. Bowen showed that as magma cools and
crystallizes, minerals form in predictable
patterns called Bowen’s reaction series.
Bowen’s Reaction Series
Bowen’s Reaction Series
• Left branch has iron-rich minerals, which
undergo abrupt changes as it cools
• Right branch represents plagioclase feldspars,
which undergo continuous change of
composition
Fractional Crystallization
• When magma cools, it crystallizes in reverse
order of partial melting
– Fractional crystallization
• As elements are removed because they
crystallize, silica is left behind
• The last elements to crystallize are potassium
feldspar and quartz (from aluminum, silica,
and potassium)